A pair of Rigger boots.
For taking kneeling shots you are allowed to support the bottom of your spine on your heel but the sole of your foot must be vertical when viewed from behind. Basically you can use your foot for support but you can't sit on it with it turned over.
Rigger boots have a steel liner in the sole and this makes them very good at giving extra support. Any boot that flexes like a trainer or a running shoe will not give this support.
A pair will cost you about £30 or so, you can spend more if you like of course. A good dose of Nikwax, not the liquid stuff but the creamy paste in a tin that stinks to high heaven, rubbed in well before you use them, and you should get suppleness and water-resistance in the uppers and support from the soles.
I've just got to go and get a replacement pair; the ones I'm chucking out I bought in the summer of 2003 and I've worn them for every shoot ever since, plus lots of other times, range maintenance, gardening etc.